This Kayak Storage Cart Build happened a few years ago. This was the first cart of its kind for my Hobie Pro Angler. The second generation with the locking industrial wheels and PVC pipes used for bunks. You MUST use the heavier wood, lag bolts, etc. Do not go cheap because your cart will not function correctly. Overall, I think this is much better than a kayak rack too due to the mobility of the cart.
My Cart Wheels - Recommended ! http://amzn.to/2EDD1nR
36" wide, 64" long. The PVC is 6 inches longer on each end. The height should be custom to the tail gate of your vehicle so that it slides on and off easily without lifting. The PVC pipes are 11 inches on center from each other.